The present invention relates to a method and a system for determining a 6-DOF-pose of an object in space. More particularly, the invention relates to a method and a system which employ one or more 2D optical-tracking markers having defined features. At least one such optical-tracking marker is attached to the object and at least one image of the optical-tracking marker is captured while the at least one optical-tracking marker is attached to the object. Using image processing technologies, 3D-position and 3D-orientation of the optical-tracking marker in space and, consequently, 3D-position and 3D-orientation of the object in space can be determined.
Determining both 3D-position and 3D-orientation of an object in space requires determining at least 6 parameters which represent 6 degrees of freedom (DOF), namely 3 parameters representing the 3D position along the three axes of a 3D coordinate system and 3 further parameters representing angular orientation about any of the three axes. Typically, the 3D coordinate system is defined relative to the at least one camera by using appropriate calibration. The 6-Degree-of-Freedom-(DOF) pose comprises both the 3D position and the 3D orientation relative to the coordinate system.
International patent application WO 2016/071227 A1, which is also assigned to the assignee of the present invention, discusses some prior art and a new approach for determining 3D position and orientation of an object in space using optical-tracking markers. According to the prior art, the optical-tracking markers often have a black-and-white chessboard pattern (checkered flag pattern), and the centroid of the marker is typically defined by an intersection of straight edges or lines formed between alternating bright and dark regions of the pattern. Typically, the centroid of the checkered pattern marker is estimated as a position of the marker within the 2D-image captured by the camera. For determining a 6-DOF-pose of an object, a plurality of markers and/or a plurality of cameras for capturing various marker images are required.
WO 2016/071227, in contrast, proposes optical-tracking markers having a substantially continuous or “analog” pattern comprising greyscale values or color-values which vary in accordance with a non-binary multitone characteristic along a circular path around the marker centroid. Image analysis of such marker patterns allows for improved accuracy. In addition, a plurality of markers can be individually coded and distinguished from one another by using different analog marker patterns. The approach of WO 2016/071227 has already proved to be successful, but there is still room for further improvement. In particular, it is desirable to further improve the accuracy in the determination of the 3D-position and 3D-orientation of an object bearing the marker. In addition, it is desirable to achieve 6-DOF-pose determination with a minimum number of markers and cameras in order to enable savings in costs and space.
A conference paper titled “Detection and Accurate Localization of Circular Fiducials under Highly Challenging Conditions” by Lilian Calvet, Pierre Gurdjos, Carsten Griwodz, Simone Gasparini from The IEEE Conference on Computer Vision and Pattern Recognition (CVPR), 2016, pp. 562-570, proposes optical-tracking markers (termed as fiducials) having a plurality of circular, concentric black-and-white rings. In one academic example, a circular marker having a single ring painted with a continuous gradation of greyscale values is examined. Paths in the marker imager are created such that the directions of path segments are given by image gradients.
The known methods and systems still leave room for improvement, especially with respect to accuracy, detection speed and costs of implementation.